Electric machines, in particular those with high power, can be equipped with superconducting rotor windings today. These kinds of synchronous machines require a cooling system in order to cool the rotor winding down to the operating temperature. A conventional specification of these kinds of cooling systems uses a so-called cooling tube in order to transfer liquid coolant from a thermosiphon system to the rotating machine and to return the evaporated coolant to the condenser. The end of a cooling tube like this, at room temperature, is conventionally introduced into the rotor of the rotating machine with a single bearing, e.g. combined with a hermetically-sealing rotary joint typically sealed with ferrofluid, or located there. The rotor is hereby designed as a hollow shaft, at least in the area of the cooling tube. The cooling tube therefore runs centrally and coaxially to the central longitudinal axis of the rotor. Between the cooling tube and the inner wall of the hollow shaft, or rather the rotor, a radial gap exists. As a rule, the cooling tube is idle and the rotor rotates around the cooling tube.
Depending on the material used and the length and diameter-dependent rigidity of the cooling tube, the free, cold end of the cooling tube which reaches into the machine or into the area of the rotor winding typically sags down a few fractions of a millimeter under its own weight, i.e. in the direction of gravitational force. In known machines, the size of the radial gap is dimensioned today in such a way that the static bending of the cooling tube in the direction of gravity does not represent a problem, i.e. the cooling tube does not touch the rotating shaft.
FIG. 2a shows in section, strongly abstracted, a section from an electric machine 102 according to the prior art, of which an axial segment of the rotor 104, which can be rotated around its central longitudinal axis 106, can be seen. A cooling tube 108, which is evacuated for thermal isolation, runs centrically in the rotor 104 along the central longitudinal axis 106. A conduit 109 is attached in the interior of the cooling tube 108, e.g. welded in firmly, for the transport of neon 110 as a coolant. The neon 110 in liquid form goes in the direction of the arrow 112 from the B-side B of the machine 102 into its interior, namely into the area 114 cooled in operation to a cryogenic temperature, the area which contains the superconducting rotor winding 116.
Cooling tube 108 and conduit 109 rest stationary relative to the surroundings, in particular relative to the rotor 104 rotating in operation. The cooling tube 108 is therefore mounted by a single gastight rotary joint 118 in the rotor so it can be rotated around the central longitudinal axis 106. The conduit 109 slopes slightly from the B-side B to the area 114 in the direction of gravitation, namely in the direction of the arrow 120. The axial section 122 of the machine 102 is at the ambient air temperature, the axial section 124 has a temperature range from the ambient air temperature of the section 122 to the cryogenic temperature of the area 114.
The cooling tube 108 is set at a distance form the inner wall 126 of the rotor 104 by a radial gap 128. Gaseous neon 110 flows against the direction of the arrow 112, out of the area 114 back to the B-side B of the machine 102, and from there to a condenser (not shown), in order to be re-liquefied there. The length of the area 124 in the axial direction of the central longitudinal axis 106 comes to approx. 0.5 to 1.5 meters.
The machine 102 is mounted on a vehicle (not shown), e.g. a ship. FIG. 2a shows the machine 102 in normal operation, i.e. when the vehicle is at rest or in quasi-static motion. The force of gravity then acts solely in the direction of the arrow 120.
However, in intended future uses for electric machines with superconducting rotors, e.g. on ships used by the military, shock loads of approx. twelve times the acceleration due to gravity, i.e. 12 g, can arise for the machine. The shock load is to be taken here as quasi-static, i.e. as though twelve times the earth's gravitational force were acting in a sustained basis. Dynamic procedures are not considered. As a result of these kinds of shock loads, the deflection of the cooling tube increases correspondingly, which would lead to a contact between cooling tube and rotating rotor in machines known today. A contact between cooling tube and hollow shaft must be avoided, however, at all costs, in order to avoid damage to the machine.
FIG. 2b shows the machine 102 from FIG. 2a under such a shock load. The conduit 109 has been left out for reasons of visibility. The force of e.g. twelve times the acceleration due to gravity 12 g acts in the direction of the arrow 120. The free end 130 of the cooling tube 108 experiences a strong deflection in the direction of the arrow 120 due to its own weight, and strikes against the inner wall 126 or even into it, which leads to substantial damage or even destruction of the machine 102. Hence the radial gap 128 disappears at the location of the impact. A deflection line 130 arises for the cooling tube 108, a line which deviates downwards from the central longitudinal axis 106, so in the direction of gravitational force of the arrow 120.
Methods of resolution available today for this problem are based on a reduction in the length of the cooling tube or in the length of the free end, and/or the enlargement of the radial gap between cooling tube and hollow shaft. However, both measures raise the undesired thermal load introduced into the area cooled to a cryogenic temperature. On the one hand, the heat conduction through the cooling tube is itself raised, namely because of the shorter overall length. On the other hand, the heat emission transferred from the warm to the cold end in the enlarged radial gap, i.e. the gap between hollow shaft (neck pipe) and cooling tube, increases. Through a larger radial gap or a shorter length of it, circulation of the working gas inside the gravity-driven convection cell established there is facilitated.
A mounting of the free end of the cooling tube in the rotor would also be possible in principle, however this end is at a cryogenically low (rotor) temperature and conventional mounting in this location is rejected. An inexpensive, direct mounting with a conventional bearing is therefore not possible. A cost-intensive mounting by means of a superconducting magnetic bearing is known, for example, from DE 103 58 341 A1.